An assay for fish lymphocytes that form rosettes with sheep red blood cells has been developed and used to measure the immune capabilities of normal and stressed animals. The technique has been used to follow the temporal response to antigen by blood and spleen lymphocytes; maximum concentrations of rosette forming cells are found at 4 days in the blood and 6 days in the spleen. Freshly caught fish responded to antigen on the above schedule while those held for 4-6 weeks did not respond well at all. Inhibition of immune response is associated with a serum factor that is transferable. Future work will characterize this factor, determine how and where it is generated, how it exerts its influence and whether its production is a non-specific response to environmental stress. Studies will continue to determine the effects of hydrocarbon stress on tissues, cells, and hemolymph of clams. Parameters measured will include histology (light and electron microscopy), histochemistry, electrophoresis and enzyme assay.